Sugars accumulation and fructokinase activity during satsuma mandarin fruit development in relation to the effect of extra nitrogenous fertilizer on the activity and expression of fructokinase were studied. The results exhibited that fructokinase activity in the tissues of edible and peel decreased during fruit development, which coincided with the accumulation of sugars, while the contents of sucrose and glucose decreased, and the activity of the enzyme increased in peel tissues of ripened fruit. After fertilizing with extra urea, the ratios of sucrose and fructose decreased in ripe fruit, while that of glucose increased compared to the control. The activity of fructokinase presented on a protein basis increased in treated fruit. Northern analysis confirmed that extra nitrogenous fertilizer enhanced the expression of Cufrk1 at the late stage of fruit development, but had no effect on Cufrk2. The results suggest that the two different genes of citrus FRK may play distinct roles in sink metabolism and Cufrk1-encoded fructokinase protein could be induced by fertilization with extra nitrogen.